1. Field of the Invention
This invention relates to leather treatment and more particularly to the upgrading of low density leather.
2. Description of the Prior Art
"Low density leather" as used herein means and refers to leather splits, whole non-bovine and reptilian leathers such as goat, pig, sheep, rabbit, raccoon, fox, snake and the like. Typically, these low density leathers have a bulk density in the range of 0.3 to 0.8 g/cc and are distinguished from top grain bovine leather which has a high density.
Top grain leather is a venier of the hide which, after removal, leaves a great deal of leather known as "leather splits." Thus, the term "leather splits" as used herein means and refers to the material remaining in a leather hide after removal of the top grain venier. The top grain is an especially valuable material due to its strength and when finished in accordance with known finishing procedures is used in the highest quality footwear. It is recognized by those skilled in the art that top grain leather can be improved. Such improvement can be conducted in accordance with the present invention by densification to improve the leather structure. For example, bovine top grain leather can be densified to a density comparable to horsehide thus providing properties comparable to a "cordovan" leather.
The present invention is applicable to leather splits from the skins of reptiles and mammals including cattle and horses and to top grain low density leather regardless of the method of tanning, whether the same be chrome-tanned, zirconium-tanned, vegetable-tanned or by synthetic tanning agents. Such tanning techniques are well known to those skilled in the art.
Leather splits have commercial application in buffed, dyed, "reverse" or suede leather products. However, the lower strength and relatively coarse texture of the split products compared to top grain leather have precluded their use broadly in shoe uppers. Due to the porosity of split leather, it cannot be finished in the same manner as top grain leather since the solution coatings coventionally used with top grain leather will penetrate split leather, resulting in a boardy product with no actual top finish.
It is important in leather products used, for example in shoe uppers, that the interior thereof absorb moisture from the foot while moisture is desirably precluded from passing from the exterior to the interior. Yet while passage of moisture is undesirable, passage of moisture vapor, that is moisture vapor permeability or breathability, of the shoe upper is recognized to be a necessary characteristic of such materials. Solution or liquid coating of split leathers, while precluding passage of moisture as liquid water, also precludes passage of moisture vapor, rendering coated products moisture vapor impermeable.
It has long been an objective of workers in the leather industry to effectively use leather splits to form a product useful in a broad range of products where top grain cowhide is the material of choice. Such efforts are exemplified by the teachings of U.S. Pat. Nos. 3,827,930 and 4,218,505, which teach laminates of leather splits and polyurethane films.
In another aspect of the invention, low density top grain leathers have been used to make gloves, handbags, clothing and the like, but are not useful as shoe uppers because they are low in integrity, memory and flex-fold characteristics. Even when low density leathers are used in the less demanding applications they do not have the long term wear characteristics of top grain cowhide. The low density characteristics of these leathers are attributable in part to the high concentration of fats in the animal hide which, during the tanning process, are removed leaving a highly porous network of loosely bonded fibers. Thus, the low density top grain leathers have reduced physical properties when compared to top grain cowhide. The configuration of fibers is not suitable for finishing to top grain uses in shoe uppers.
In yet another aspect, when low density top grain leathers are used in clothing, the thickness should be as low as possible to render a garment which has good hand and drape. However, sufficient thickness must be maintained to provide a garment with some integrity.
In accordance with the present invention, a process is provided wherein the physical and chemical properties of low density leathers are upgraded.
Further, in accordance with the present invention, leather splits are upgraded to provide a product having all the advantages of top grain cowhide.